The Physics of the Universe - Difficult Topics Made Understandable
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A precocious child might write his or her full address as Main Street, Toronto, Canada, the Earth, the Solar System, Orion Arm, the Milky Way, the Local Group, the Virgo Supercluster, the Universe.

The Solar System consists of the Sun and those objects bound to it by gravity (the terrestrial planets, Mercury, Venus, Earth and Mars; the gas giants, Jupiter, Saturn, Neptune and Uranus; and various dwarf planets, proto-planets and asteroids). However measured, it is less than a light year across.

The Milky Way galaxy is a barred spiral galaxy with a diameter of about 100,000 light-years and containing about 200 billion stars. Our Solar System is located towards the edge of one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur, about 25,000 to 28,000 light years from the galactic center.

The Local Group is a small group or cluster of gravitationally-bound galaxies, which includes the Milky Way, the Andromeda galaxy and the much smaller Triangulum galaxy, along with smaller satellite and dwarf galaxies such as the Large Magellanic Cloud, the Sagittarius Dwarf Galaxy and Canis Major Dwarf Galaxy.

The Virgo Supercluster is an irregular group of clusters of galaxies, between 100 and 200 million light years in diameter, which incorporates our Local Group of galaxies and about 100 other clusters. The Local Group is located in a small filament on the outskirts of the supercluster. It is thought that superclusters may also be arranged in even larger structures called walls (such as the Sloan Great Wall, which is about 1.5 billion light years long), although these may not be true structures as their parts are not gravitationally bound together.

The universe is what we usually think of as the totality of known or supposed objects and phenomena throughout space. The observable part alone contains over ten billion trillion stars arranged in about 100 billion galaxies, and is estimated to be around 93 billion light years in diameter. By definition, we are at the center of our observable universe, but it is totally unknown where we are in the universe as a whole.

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